Hi.
I am looking for a certain max plugin which helps you with manifold checking and other stuff. There was a link here somewhere on this site but I cannot find it back. So for future searches I hope this thread will be found with the plugin mentioned below. Thanks in advance.

Yes, the manifold check is weak in speed but you usually just need to check the lowpoly beginning mesh. The resulting object should stay being a manifold if not heavily altered (deleting polies, sewing edges,etc.).

Yeah, I use STL check all the time, and have never had issues as long as I fix anything it shows me. It doesn't list "manifold" (in my Max version anyway) but if you select all errors, it's obvious what to fix afterwards. I've written a small script that does a weld verts, and STL check all in one, and I use it all the time, works great.